1. Field of the Invention
This invention relates to communication systems, and more specifically to a system capable of transmitting voice information and data information over a single pulse code modulation channel.
2. Description of the Prior Art
Systems employing pulse code modulation (PCM) are well-known in the prior art. For example, telephone communication systems often utilize pulse code modulation, in which an analog voice signal is converted to a series of binary pulses. Such telephone communication systems utilize a fixed sampling rate, commonly eight (8) kilohertz. By converting the sampled speech signal to an eight (8) bit word, the digitized speech is transmitted by pulse code modulation at 64 kilobits per second. Prior art speech transmission utilizes a fixed sampling rate, due to the fact that prior art filters utilize RC circuits to provide the filter characteristics. Utilizing fixed resistors and capacitors, the characteristics of prior art filters are also fixed.
Data transmission, such as from a computer or microprocessor, also utilizes a series of binary pulses. One common data transmission rate is 9.6 kilobits per second.
Prior art methods of transmitting both speech and data require either separate voice and data transmission channels, or a single channel of increased bandwidth capable of carrying both data and speech. Thus, for a voice transmission of 64 kilobits per second to be combined with a data transmission of 9.6 kilobits per second, a transmission channel capable of carrying 73.6 kilobits per second is required. In many instances this is not practical or possible. For example, standard PCM channels and equipment are designed to be capable of handling 64 kilobits per second, and it is not possible to transmit information in excess of this rate, without redesigning the receiving equipment.